The present application claims priority of Chinese patent application Serial No. 20061001 1943.X, filed May 19, 2006, the content of which is hereby incorporated by reference in its entirety.
As the requirement for a security inspection system such as at Customs is constantly increased, the relevant technology has been widely applied as in U.S. Pat. No. 5,044,002, in which X-rays having two different energy levels are utilized to perform non-destructive inspection on an object while the material of the object is identified. Recently, the dual-energy method is resumed to implement material identification within high-energy range (>1 MeV) in the non-destructive inspection of large-sized objects, as disclosed in U.S. Pat. No. 5,524,133.
The physical principle of the dual-energy method for discriminating material is that when two X-ray beams having different energy levels interact with the same object, since the photon energy levels of the two beams are different from each other, there exists a difference between their interactions with the object. Such difference as a whole can simply be represented by the difference in attenuation index. Based on such principle various methods of alternately generating X-rays having two energy levels have been proposed, such as in U.S. Pat. No. 6,069,936 and international application WO 00/43760 there is disclosed a single radiation source which modulates a high energy spectrum by means of material absorption. In addition, international application WO 2004/030162 A2 discloses a method of alternately generating X-rays having high and low energy spectra by an accelerator. However, when the alternately generated X-rays having high and low energy spectra are utilized to scan an object, a severe defection occurs as follows. Since the X-rays having two energy levels are alternately generated at certain frequency, there is certain time interval between the generation of each ray. The inspected object always move at certain speed, and thus it will move by some distance during the time interval between the generation of X-rays having high and low energy levels. Therefore, when used to scan the inspected object (e.g., luggage, container, etc.), the interactions between the two kinds of X-rays and the object are not completely identical. This will have a negative impact on the discrimination accuracy, especially at the edge of the inspected object where the rays having two energy levels may interact with different objects, thereby incurring a false discrimination result. Meanwhile, in order to suppress the error due to rays having high and low energy levels interacting with different positions, the conventional method is to slow down the movement of the inspected object. This method severely limits the efficiency of object inspection and can't solve the false discrimination occurring at the edge of the object.